Electromagnetic radiation is observed when an electron beam is injected into a uniform quiescent magnetoplasma. The emission peaks near the electron plasma frequency ($\omega \gtrsim {\omega }_p\gg {\omega }_c$) and is found to result from the scattering of unstable intense electrostatic plasma waves off self-consistently produced ion acoustic waves with wave vectors ${\mathcal{k}}_i\simeq {\mathcal{k}}_e$. The emission is localized, polarized, and negligible in intensity at $\omega =2\mathrm{}{\omega }_{\mathrm{pe}}$. Space-time growth and $\omega -\mathcal{k}$ properties of the three-wave interaction are presented.

• Received 3 June 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.95